Method of generating unipolar and bipolar pulses

ABSTRACT

The method for generating bipolar and unipolar mechanical pulses is described. The unipolar pulses can be in a form of a single unipolar pulse or pairs of unipolar pulses of opposite polarity.

United States Patent Dixon [45] Apr. 11, 1972 [54] METHOD OF GENERATING UNIPOLAR 2,562,450 7/1951 DeLano, Jr. ..3l0/8.1 x AND BIPOLAR PULSES 2,398,701 4/1946 Firestone .1 X 2,920,192 1/1960 Peck .l X 1 Invent Norman DIXOII, Pam, Wash- 3,114,058 12/1963 Lyth [73] Assignee: The United States of America as 3,202,868 8/1965 Blank represented by the United states Atomic 3,004,424 10/1961 Henry Energy Commission Primary Examiner-J. D. Miller [22] Wed: 1971 Assistant Examiner-Mark O. Budd 2 App] 111,957 AttorneyRoland A. Anderson [57] ABSTRACT [52] US. Cl ..3l0/8.l [51] Int. Cl. ,H01 7/00 The method for generatmg blpolar and unipolar mechanical [58] Field of Search ..310/8.1; 340/8; 73/67.8 R; pulses is described- The unipolar Pulses can be in a form of a 333/30 single unipolar pulse or pairs of unipolar pulses of opposite polarity. [56] References Cited 9 C 15 Drawing Figures UNITED STATES PATENTS 2,949,028 8/1960 Joy ...310/8.l Q; l

Patented April 11, 1972 3,656,012

6 Sheets-Sheet l Fig- PRES S URE EI-Z1 25 METHOD OF GENERATING UNIPOLAR AND BIPOLAR PULSES CONTRACTUAL ORIGIN OF THE INVENTION BACKGROUND OF THE INVENTION The invention described herein relates to the generation of mechanical pulses in the form of unipolar and bipolar pulses. In the field of nondestructive testing much effort has gone into improving range resolution, broad band spectral content, penetration capabilities and eliminating near field effects. Innovations such as destructive lenses, matched damping members, single surface operation of very thick transducer elements, broad band amplifiers, frequency diffraction gratings and electronic enhancement of the rising slope of received signals have been developed for obtaining improved resolution response. However, relatively little work has been done to optimize the transmitter pulse for improved response.

Various types of ultrasound pulses have been generated for nondestructive testing. For example, output pulses having a rise time faster or slower than the transducer element half period resonance with exponential decay times have been used. For the faster rise time the transducer rings with a decay rate proportional to the ratio of the transmitter rise time to the transducer half period and the transducer mechanical damping or backing member. For the slower case a low-frequency component is imposed on the leading portion of the pulse and the transducer element resonates with a decay rate similar to that of the fast rise time pulse. These pulses have relatively poor range resolution and low or high spectral band width depending on whether the band width is evaluated from the viewpoint of narrow or broad spectral band width application. These pulse techniques also have relatively poor flaw separation resolution and can miss significant flaws because of interference, orientation and ultrasonic beam field effects.

A brute force fast rise transmitter pulse applied to a highly damped destructivelens transducer element produces an output pulse which is nearly a sinusoidal single cycle with very little resonance ringout. Using a pulse of this nature, resolution down to 0.010 inch longitudinal range resolution in steel has been obtained. This pulse technique has the advantage of high range resolution, relatively broad spectral range and minimization of near field effects. It has disadvantage in that the pulse is difficult to reproduce, particularly with different transducers and pulse systems. The equipment using this system is expensive and this system will not develop unipolar pulses. The receive response characteristics are also not ideal for varying signal types.

Variable length sinusoidal transmitter pulse trains tuned to transducer resonance are also used. This type of pulse is used primarily for attenuation measurement at a single frequency or may also be used for thickness measurements employing destructive resonance. Stable variable-frequency transmitters which are relatively expensive are required for this pulse type. This type wave propagation can be up to continuous and transmitters and transducers can be frequency modulated to cover relatively narrow frequency bands of spectral information. There is, however, very little range resolution using this pulse and the spectral power density is very narrow.

It is therefore an object of this invention to provide an excitation signal for developing a single unipolar pulse from a transducer.

Another object of this invention is to provide an excitation signal for developing a single bipolar pulse from a transducer.

Another object of this invention is to provide an excitation signal for developing a pair of unipolar pulses of opposite polarity from a transducer.

Another object of this invention is to provide signals for developing mechanical pulses having improved range resolution.

Another object of this invention is to provide signals for developing mechanical pulses which have a predictable broad spectral power density distribution.

Another object of this invention is to provide signals for developing mechanical pulses which have little or no transducer beam field effects.

Another object of this invention is to provide signals for developing mechanical pulses which can be approximated by straight lines so that they can easily be used in computer analysis.

SUMMARY OF THE INVENTION In practicing this invention, a method is provided for generating mechanical pulses. An excitation voltage is applied to a transducer to charge the transducer. The amplitude of the voltage increases from an initial value to a predetermined value at a time rate of change which inhibits measurable mechanical pulse generation by the transducer. After the excitation voltage has reached the predetermined value, the voltage is decreased linearly to the initial value during a specific time period to develop a single pulse having a period equal to the resonant sinusoidal period of the transducer. This excitation method develops a single unipolar pulse which is used for nondestructive testing of materials.

The time period required to develop the unipolar pulse is dependent on the backing impedance of the transducer. If the backing impedance is matched, the period of the excitation voltage is one-half the resonant sinusoidal period of the transducer. If the backing impedance is zero or unmatched, the period of the excitation voltage is equal to the resonant sinusoidal period of the transducer. Where the backing impedance is not zero and is unmatched, a dual slope excitation voltage is used.

A bipolar pulse having similar characteristics can also be developed by using an excitation voltage which increases in amplitude from the initial value to the predetermined value in the required time period and then decreases to the initial value in the same period. A pair of unipolar pulses can be developed by increasing the amplitude of the excitation voltage from the initial value to the predetermined value in the required time period to produce the first pulse. The excitation voltage is maintained at the predetermined value for a time period sufficiently long to permit ultrasound generated by the transducer to be reduced below a particular magnitude. The excitation voltage is then reduced to the initial value in the required time period. In this manner, a pair of unipolar pulses of opposite polarity are developed. In the time period between the unipolar or bipolar pulses generated in this manner, a receiver may be connected to the transducer for receiving reflected energy for analysis.

DESCRIPTION OF THE DRAWINGS The invention is illustrated in'the drawings, of which:

FIG. 1 is a curve showing the unipolar pulse;

FIG. 2 is a block diagram showing the equivalent circuit of a transducer;

FIGS. 3, 4 and 5 show the output forces developed by excitation of the transducer equivalent circuit of FIG. 2;

FIG. 6 shows the construction of a unipolar pulse in an air backed transducer;

FIG. 7 shows the unipolar response of a transducer with matched backing impedance;

FIG. 8 shows the dual slope ramp excitation required to develop unipolar pulses from a transducer with unmatched backing impedance other than zero;

FIGS. 9, 10 and 12 show the voltage applied to a transducer to develop desired mechanical pulses;

FIGS. 11 and 13 show the output pulses from a transducer as a result of the voltages shown in FIGS. 10 and 12;

FIG. 14 is a curve showing the power spectral density resulting from the mechanical pulses of FIGS. 1 and 11; and

FIG. 15 is a curve showing the power spectral density resulting from the mechanical pulse of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown the waveform of a unipolar pulse which is a single triangular pressure wave radiated from a properly excited transducer. The width, T, of the pulse is equal to one period of the transducers natural resonant frequency. The pressure amplitude of this pulse will be of the same order of magnitude as the maximum half-cycle of an exponentially damped oscillatory burst resulting from rapid excitation of the same transducer.

To explain the generation of the unipolar pulse, reference is made to an equivalent circuit of a piezoelectric plate transducer, FIG. 2. The body of the transducer is represented by a three terminal delay line whose time delay is equal to one-half the transducers period at resonance. The velocity of propagation of a mechanical wave in the delay line is v. The lens and backing member impedances are represented respectively by Z and Z Z, is the source impedance and V is the electrode voltage. C, is the electrical capacitance at the terminals and Z, is the characteristic impedance of the delay line (analog of piezoelectric materials acoustic impedance). X is the thickness of the transducer and z is a transfonnation ratio. The voltage appearing at Z is the analog of ultrasonic force (pressure 1: surface area) radiated into the lens, while that appearing at Z is the analog of force radiated into the backing member.

At ultrasonic frequencies a piezoelectric transducer is effectively damped by its mass with the exception of areas immediately adjacent to the electrodes. Therefore the only sources of acoustic waves are at the front and back surfaces. Each surface will radiate waves in both the positive and negative X directions. The initial response to a positive Dirac voltage impulse applied at t= is shown in FIG. 3 with forces (F) labeled. Upon application of the voltage a positive and negative force will be generated at each surface.

The complete impulse response, FIG. 4, of the transducer can be derived through a transmission line treatment of succeeding reflections and radiations in the delay line. Referring back to the circuit of FIG. 2, the reflection coefficient at the front face is 0 =(Z1" Z0)/(ZI Z0) while that at the rear face is After the initial front face radiation F at t 0, no signal leaves the transducer until the force F originally generated at the rear face reaches the front face. A portion of this wave will leave the front face with magnitude (1 7 F That portion of the wave not radiated will be reflected back into the transducer with magnitude T F By subjecting all signals generated to a similar transmission line treatment, the complete train of impulses leaving the front face can be drawn. This impulse train is shown in FIG. 5.

The simplest case for unipolar pulse generation employs an air backed transducer with a water or other low impedance lens medium at the front face. Practical values for this case in an actual transducer are Z 1/20 2,, and Z 0. The driving function required to generate a unipolar pulse in an air backed transducer is a linear voltage ramp. The ramp starts at t O and at t= 2X /v. The duration of the ramp is critical if a clean baseline after t= 2X/v is to be obtained. The amplitude of the unipolar pulse will be directly proportional to the slope of the ramp.

The combination of force pulses responsible for generation of the unipolar pulse in this special case is shown in FIG. 6. Ramp waves of force are radiated from the front face with amplitudes as predicted by the impulse diagram of FIG. 6. At t 0 a positive force or compression ram is radiated in phase with the driving voltage. Its amplitude is established by the values of Z, and Z, to be =(hC, V/20) where V is the time function of voltage describing the input ramp and h is the piezoelectric constant. At t= X/v, the rarefaction force wave F generated at the rear face reaches the front face and will be radiated with amplitude =(2hC V/20). At t 3X/v, force F will have made a round trip in the transducer and will be radiated with approximate amplitude 2hC V/20. The complete series of radiated pulses can be derived in this manner. The algebraic sum of all these is the total force radiated with time from the front face. As can be seen from FIG. 6, this summation of forces is a unipolar pulse with nearly total cancellation of all radiated signals after t= 2X/v.

FIG. 7 shows the models response for matched backing, i.e., Z Z In this case the required ramp length is only X/v. This correlates with theory since the unipolar pulse will be the algebraic sum of only two pressure ramps, F and (l 1- F,,, which are equal and out of phase for any value of Z,. If they are to generate a unipolar pulse the ramp must terminate at t= X/v.

The equivalent circuit of FIG. 2 with linear ramp excitation provides a unipolar output for only the two cases 2 very small, Z 0 and Z any value, Z Z In practice, many transducers have epoxy backing of nonzero impedance and cannot be excited in the unipolar mode with a single linear ramp.

It has been found that a transducer can generate unipolar pulses where the nonmatched backing impedance is not matched or zero if certain conditions are met. These are: (A') that the ramp has a dual slope with a breakpoint at t X/v; (B) the two ramp slopes are of a specific ratio determined by transducer constants; and (C) the impedances Z and Z are equal. Such an excitation ramp is shown in FIG. 11.

By requiring that the time derivative of radiated signals occurring in both even and odd time intervals after t 2X/v is zero, the following relationships hold:

For odd intervals of X/v K2 Z0) k 0 and for even intervals 2 2 ZO/ZI 0) 1 0 where k, and k are the ramp slope coefficients as shown in FIG. 8, and a is the attenuation factor for one trip across the transducer in percent x 100. For both of these relationships to be true simultaneously, Z must equal Z The ramp ratio can then be determined to be 2/ l 0 2/ o 2)- Referring to FIG. 9, there is shown the waveform of a voltage pulse which can be applied to a transducer to develop a unipolar mechanical pulse therefrom. The transducer is charged by a linear ramp voltage 10 which gradually increases from an initial value to a final value 11. The time period over which this increase takes place A is long enough so that the transducer is charged without exciting the transducer to develop mechanical output pulses. After the transducer is charged, the voltage applied to the transducer is reduced to the initial value as shown by the linear ramp voltage 12. This reduction takes place over time period C which is equal to the time period required to develop a full single-cycle sinusoidal output pulse from the transducer.

Time C is variable depending upon the construction of the transducer. For example, an air backed transducer time period C is equal to the full single-cycle sinusoidal period of the transducer 2X/v. With a matched back transducer, the time period C is one-half the full single-cycle sinusoidal period X/v. Where the backing is neither matched nor zero (air, for example), the dual slope signal of FIG. 8 may be used. In either example, time period C is of the proper length to develop the single-cycle sinusoidal output pulse from the transducer. Time period B is arbitrary in length and may be zero, so that the output pulse is developed as soon as the transducer is charged. During the time period E the transducer is connected to a receiver to receive the reflected pulses for analysis. FIG. 1 shows the shape of the transmitted mechanical pulse. The time period T is equal to the full single-cycle sinusoidal period of the transducer.

Referring to FIG. 10, there is shown a waveform which will excite the transducer to develop two unipolar pulses. A linear ramp voltage (or dual linear ramps as required) 15 is applied to the transducer for a period F. When the ramp voltage 15 reaches a predetermined voltage level 16, it is maintained at this level for a time period G. At the end of time period G the voltage applied to the transducer is reduced to the initial value during a time period F at the rate shown by linear ramp voltage 17. (Linear ramp voltage 17 may be a dual linear ramp.) Periods F are of the proper duration to develop the normal full single-cycle sinusoidal pulses from the transducer. Period G is an arbitrary time period during which the initial pulse developed by the linear ramp voltage is transmitted and received. It should also be sufficiently long so that the transducer is quiescent prior to applying ramp 17.

Referring to FIG. 11, there is shown the transmitted pulses from the transducer for the excitation signal shown in FIG. 10. Each of the pulses has a time period T which is equal to the full single-cycle sinusoidal period of the transducer.

Referring to FIG. 12, there is shown the required excitation voltage to develop a single bipolar pulse. The linear ramp voltage 20 rises from the initial value to the predetermined value during a time period I and is immediately reduced to the initial value according to ramp voltage 21 in a time period I. (Ramps 20 and 21 may be dual linear ramps as required.) Time periods I are of the proper time duration to develop a full single-cycle sinusoidal output pulse from the transducer. Referring to FIG. 13, there is shown the output pulse from the transducer. In this example, the time period T is equal tothe full single-cycle sinusoidal period of the transducer.

FIG. 14 illustrates the spectral power density for a 15 MHz unipolar pulse of the kind shown in FIGS. 1 and 11. It can be seen that the power density covers a broad range. In FIG. 15 there is shown the spectral power density for the bipolar pulse of FIG. 13.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of generating mechanical pulses from a transducer coupled to a load including the steps of:

a. applying to said transducer an excitation voltage changing in amplitude from an initial value to a predetermined value during a first time period,

b. maintaining said excitation voltage at said predetermined value during a second time period, and

c. changing said excitation voltage across said transducer to a value proximate said initial value during a third time period, said excitation voltage during said third time period being formed of at least one and no more than two linear ramps, said third time period and said ramps being such that the amplitude of the signal from the transducer rear face reaching the load is substantially equal to the magnitude of the second and subsequent cycles of the signal from the transducer front face reaching the load and further is 180 out of phase with said second and subsequent cycles.

2. The method of generating mechanical pulses from the transducer of claim ll wherein:

a. the characteristic impedance of the backing material of said transducer is substantially zero,

b. said excitation voltage is applied to said transducer during said first time period to change the amplitude from said initial value to said predetermined value at a time rate of change to inhibit generation of mechanical pulses from said transducer, and

c. said excitation voltage across said transducer during said third time period is a single linear ramp with a duration equal to the resonant sinusoidal period of said transducer.

3. The method of generating mechanical pulses from the transducer of claim 2 wherein:

a. said second time period is zero.

4. The method of generating mechanical pulses from the transducer. of claim 1 wherein:

a. said excitation voltage across said transducer during said first time period is a single linear ramp with a duration equal to the resonant sinusoidal period of said transducer, and

b. said second time period during which said excitation transducer of is maintained at said predetermined value is sufficiently long to permit mechanical pulses thereby generated to be reduced below a particular magnitude.

5. The method of generating mechanical pulses from the transducer of claim 4 wherein:

a. said second time period is zero.

6. The method of generating mechanical pulses from the transducer of claim 1 wherein:

a. The characteristic impedance of the backing material of said transducer is substantially equal to the characteristic impedance of said transducer,

b. said excitation voltage is applied to said transducer during said first time period to change the amplitude from said initial value to said predetermined value at a time rate of change to inhibit generation of mechanical pulses from said transducer, and

c. said excitation voltage across said transducer during said third time period is a single linear ramp with a duration equal to one-half the resonant sinusoidal period of said transducer.

7. The method of generating mechanical pulses from the transducer of claim 6 wherein:

a. said second time period is zero.

8. The method of generating mechanical pulses of claim 1 wherein:

a. said excitation voltage across said transducer during said third time period is formed of two linear ramps with the duration of each of said ramps being equal to one-half the resonant sinsuoidal period of said transducer, and

b. the ramp ratio k /k is found by the formula 2/ 1 0 Z2/Z0'I'Z2), where k and k are the ramp slope coefficients of each of the two linear ramps, 0- is the attenuation factor for one trip of a pressure wave across the transducer, Z is the characteristic impedance of the transducer and Z is the impedance of the transistor backing material.

9. The method of generating mechanical pulses of claim 8 wherein:

a. said second time period is zero,

b. said excitation voltage across said transducer during said first time period is formed of two linear ramps with the duration of each of said ramps being equal to one-half the resonant sinusoidal period of said transducer, and

c. the ramp ratio K lk is found by the formula 2/ l o 2/ 0 2) where k, and k are the ramp slope coefficients, 0' is the attenuation factor for one trip of a pressure wave across the transducer, 2 is the characteristic impedance of the transducer, Z is the impedance of the transistor backing material. 

1. A method of generating mechanical pulses from a transducer coupled to a load including the steps of: a. applying to said transducer an excitation voltage changing in amplitude from an initial value to a predetermined value during a first time period, b. maintaining said excitation voltage at said predetermined value during a second time period, and c. changing said excitation voltage across said transducer to a value proximate said initial value during a third time period, said excitation voltage during said third time period being formed of at least one and no more than two linear ramps, said third time period and said ramps being such that the amplitude of the signal from the transducer rear face reaching the load is substantially equal to the magnitude of the second and subsequent cycles of the signal from the transducer front face reaching the load and further is 180* out of phase with said second and subsequent cycles.
 2. The method of generating mechanical pulses from the transducer of claim 1 wherein: a. the characteristic impedance of the backing material of said transducer is substantially zero, b. said excitation voltage is applied to said transducer during said first time period to change the amplitude from said initial value to said predetermined value at a time rate of change to inhibit generation of mechanical pulses from said transducer, and c. said excitation voltage across said transducer during said third time period is a single linear ramp with a duration equal to the resonant sinusoidal period of said transducer.
 3. The method of generating mechanical pulses from the transducer of claim 2 wherein: a. said second time period is zero.
 4. The method of generating mechanical pulses from the transducer of claim 1 wherein: a. said excitation voltage across said transducer during said first time period is a single linear ramp with a duration equal to the resonant sinusoidal period of said transducer, and b. said second time period during which said excitation transducer of is maintained at said predetermined value is sufficiently long to permit mechanical pulses thereby generated to be reduced below a particular magnitude.
 5. The method of generating mechanical pulses from the transducer of claim 4 wherein: a. said second time period is zero.
 6. The method of generating mechanical pulses from the transducer of claim 1 wherein: a. The characteristic impedance of the backing material of said transducer is substantially equal to the characteristic impedance of said transducer, b. said excitation voltage is applied to said transducer during said first time period to change the amplitude from said initial value to said predetermined value at a time rate of change to inhibit generation of mechanical pulses from said transducer, and c. said excitation voltage across said transducer during said third time period is a single linear ramp with a duration equal to one-half the resonant sinusoidal period of said transducer.
 7. The method of generating mechanical pulses from the transducer of claim 6 wherein: a. said second time period is zero.
 8. The method of generating mechanical pulses of claim 1 wherein: a. said excitation voltage across said transducer during said third time period is formed of two linear ramps with the duration of each of said ramps being equal to one-half the resonant sinsuoidal period of said transducer, and b. the ramp ratio k2/k1 is found by the formula k2/k1 sigma (Z0 - Z2/Z0 + Z2), where k1 and k2 are the ramp slope coefficients of each of the two linear ramps, sigma is the attenuation factor for one trip of a pressure wave across the transducer, Z0 is the characteristic impedance of the transducer and Z2 is the Impedance of the transistor backing material.
 9. The method of generating mechanical pulses of claim 8 wherein: a. said second time period is zero, b. said excitation voltage across said transducer during said first time period is formed of two linear ramps with the duration of each of said ramps being equal to one-half the resonant sinusoidal period of said transducer, and c. the ramp ratio K2/k1 is found by the formula K2/k1 sigma (Z0 - Z2/Z0 +Z2), where k1 and k2 are the ramp slope coefficients, sigma is the attenuation factor for one trip of a pressure wave across the transducer, Z0 is the characteristic impedance of the transducer, Z2 is the impedance of the transistor backing material. 